Allow for optional FedEx package label stock type (label dimensions)

[adacosta]

Currently, a hard coded value of 'PAPER_7X4.75' is supplied for every package label. The FedEx WebServices Developers Guide says:

When using an ImageType of PDF or PNG, these values display a thermal format label:
  PAPER_4X6
  PAPER_4X8
  PAPER_4X9
these values display a plain paper format shipping label:
  PAPER_7X4.75
  PAPER_8.5X11_BOTTOM_HALF_LABEL
  PAPER_8.5X11_TOP_HALF_LABEL

I've allowed for the default (no option specified) to retain the current functionality. In other words, 'PAPER_7X4.75' will be used without a specified option.

In addition to the automated tests, I've manually tested with 'PAPER_4X6' (what I need for a thermal printer)

Sample use:

carrier.create_shipment(location_1, location_2, [package], test: true, label_stock_type: 'PAPER_4X6')

[jonathankwok]

Hey, thanks for putting this PR out! Everything looks great, I can merge it in for you.

CC: @mdking

[adacosta]

thanks @jonathankwok

[mdking]

👍